I've pretty much stayed out of this conversation because it is about Cancún, and I haven't been there in years. So realize anything I say is general, and more about my experience in Mazatlán than Cancún.
Cancún's tourist areas are more "Americanized," so many people don't realize the poverty that lies below the surface. Your maid really needs the tips people leave. The timeshare touts and the front desk people earn more, because they speak English. The more English they speak, the more they make (although I've never known a front desk person in Mexico to earn anything near $100/week!).
School supplies are a good gift, as are new and gently worn clothes for young children. If the resort can't point you to an orphanage to donate this to, a local church could, I am sure.
As for the Walmart card. Its a generous gesture, one which would be appreciated here I am sure. But in Mexico, mainly middle class people shop at Walmart. Walmart in Mexico is about half a grocery store, half other stuff. I would doubt that maids shop there. I've been to Walmart and Sams in Mazatlán, and that's what I've observed. Think about the prices at the Walmart at home, and the ones in Mexico are about the same...good prices by our standards, yes. but probably the things would be a splurge for the maids, and not something she'd spend money on.
We think that its ackward to give people money, or thoughtless sometimes, but in this case money will go a lot further than gift cards, so its the kindest thing to do.
I speak some Spanish, so if I am checking out early I ask my maid if she works that day (so if she doesn't I make arrangements to give her the final tip early). I also belive in tipping early and often, so I give her a tip the first time I see her, and then a bit more midweek, too. I way overtip, but that's ok, I blow more money than this on inconsequential things at home.
Fern